In accordance with one aspect of the present invention, an astronomical imaging array is formed of several widely spaced photon collectors (e.g. photodiodes). Each collector has associated with it a digitizing sampler that collects a stream of sample data from the photodiode in response to a trigger signal provided by a time source. Samplers at different photon collectors are triggered at different instants in accordance with their spacing, and their relative optical path differences from the object being imaged. In particular, each sampler is triggered to collect a record of samples when a given phase front of light from the object being imaged is expected to pass the photon collector.
Using intensity interferometry techniques, the sampled data from each photon collector is correlated with data from other collectors, yielding a waveform whose individual values represent a brightness line integral through the object. Using different photon collector pairs, different sets of brightness line integrals through the object are produced. Matrix algebra is then employed to synthesize the collected set of line integrals into a two dimensional image representing the brightness of the object being imaged.
Intensity interferometry was invented by Hanbury Brown and Twiss, and is exemplified by the following articles (all by Hanbury Brown et al): "Correlation Between Photons in Two Coherent Beams of Light" (Nature, Vol. 177, pp. 27-29, Jan. 7, 1956); "A Test of a New Type of Stellar Interferometer on Sirius," (Nature, Vol. 178, pp. 1046-48, Nov. 10, 1956); "The Stellar Interferometer at Narrabri Observatory; I: A Description of the Instrument and the Observational Procedure" (Monthly Notices of the Royal Astronomical Society, Vol. 137, pp. 375-392, 1967); and "The Angular Diameters of 32 Stars," (Monthly Notices of the Royal Astronomical Society, Vol. 167, pp. 121-136, 1974). (These articles, and others landmarks in the field of interferometry, are reprinted in the recent volume "Selected Papers on Long Baseline Stellar Interferometry," SPIE Milestone Series, Vol. MS 139, 1997, edited by Lawson.)
No one, to our knowledge, has employed Hanbury Brown-Twiss (HBT) techniques as the basis for an imaging instrument. Instead, astronomical use of such techniques (for forty years) has been limited to the determination of stellar diameters.
We believe one factor contributing to the failure of others to employ HBT techniques in imaging applications may be certain interpretations that are traditionally accorded the original Hanbury Brown papers. We note some possible alternative interpretations that may be central to the functioning of the below-described embodiments.
One area in which HBT's work may have been misunderstood is the common impression that intensity interferometry requires that the two sensors be equidistant from the object being measured (c.f. section 3.3 of their 1967 paper). HBT state that correlation diminishes about 10% with the first 1 nanosecond delay between the two collectors (corresponding to a path length difference of about one foot), and diminishes exponentially as this delay is increased.
(To provide this equidistant spacing, the observatory at Narrabri employed two detectors on a circular track having a radius of about 100 m. By this arrangement, the two detectors could be placed equidistant from the star being measured, while also providing a range of spacing from 0-200 meters.)
We presently believe that intensity interferometry techniques can be used with arbitrarily positioned detectors, including arbitrary three dimensional arrangements (e.g. one or more in space).
Another possible misinterpretation of HBT's original findings is the impression that the variation in correlation, as a function of detector spacing, is a single-lobed function (c.f. FIG. 5 of HBT's 1967 paper), trailing to zero at a normalized spacing of about 3.5. We believe it is likely that this function instead exhibits multiple side lobes, of diminished amplitude, extending far beyond the 0-3.5 range contemplated by HBT. (Our belief is based, in part, on Fraunhofer analysis of the stellar disk being imaged.) The existence of such secondary lobes would begin to suggest that detectors can be spaced much further apart than previously thought possible.
We caution that our foregoing critiques of the HBT work are preliminary but are believed to explain the operation of our detailed embodiment. Further study, however, may reveal other or additional rationales.
Throughout our work we encounter wave/photon conundrums. For example, classical photon theory holds that a single photon can only be sensed once, e.g., when it kicks an electron out of a valence band in a photodiode. Thereafter, it ceases to exist. Yet HBT interferometry seems to illustrate the contrary, by evidencing intensity correlations between spaced-apart optical detectors.
Hanbury Brown and Twiss acknowledged these conundrums, but posed no answer. In the intervening forty years, no satisfactory resolution of the conflicting photon and wave theories has been found. We offer none, and instead rely exclusively on classical wave theory in analyzing operation of our system.
So that the present invention may be better appreciated, it may be helpful to review other work in the field of astronomical interferometry.
Most astronomical interferometry traces its origins back to Michelson's work in the late 1800s. Michelson showed that light from a single source, traveling different paths, can be combined to produce fringe patterns. This is the principle on which modern radio telescope arrays work. For example, the New Mexico Very Long Array (VLA) includes 27 antennas (each with a 25 m reflector) movably positioned within a Y-pattern of up to 22 miles across. The data from each pair of telescopes is combined (often after recording on tape with a time synchronization signal) to form interference patterns. The structures of these patterns, and their changes with time (as the Earth rotates) reflect the structures of radio sources in the sky. By applying Fourier techniques to the resulting interferometric patterns, conventional imagery can be produced. (Such radio astronomy interferometry/imaging is well detailed in extensive literature familiar to those active in the field. A bibliography of writings on the topic can be found at the internet address http://sgra.jpl.nasa.gov/mosaic.sub.-- v0.0/Spacevlbi.sub.-- lib.html.)
The resolution of imagery obtained by such interferometric techniques depends on the size of the array (the baseline). The fully extended VLA, for example, has a resolution of 0.04 arcseconds at 43 GHz. To obtain still better resolution, longer baselines are required. An example of a transcontinental baseline is the Very Long Baseline Array, which employs ten identical radiotelescopes spread from Hawaii to the U.S. Virgin Islands.
A decade ago, first steps were made to extend interferometric baselines still further--into space. During 1986-1988, the NASA TDRSS satellite observatory--working in conjunction with ground-based telescopes in Japan and Australia--proved the feasibility of such systems by recording interferometric fringes from six radioemitting sources.
Several space VLBI (very long baseline interferometry) projects have since been proposed but have not been completed, among them the IVS and QUASAT programs. The costs and difficulties associated with placing complex optical telescopes into orbit played large roles in the demise of these projects.
Just recently, the first fully-operational space VLBI project began operation: the Japanese Institute of Space and Astronautical Science's VSOP mission ("VLBI Space Observatory Program," space antenna deployed Feb. 27, 1997, first fringes produced May, 1997). Another imminent program is the Russian RadioAstron mission, developed by the Astro Space Center of the Lebedev Physical Institute and scheduled for launch later in 1998. Each of these programs utilizes a single 8-10 meter radio telescope in an elliptical Earth orbit, in conjunction with ground radiotelescopes. Each observes in the 22, 5, and 1.6 GHz bands. (Imagery from, and information about, the VSOP mission is publicly available on the world wide web at http://www.vsop.isas.ac.jp/. Information about the RadioAstron mission is publicly available at http://sgra.jpl.nasa.gov/mosaic.sub.-- v0.0/RadioAstron.html.)
(The movement of an antenna in a space VLBI array, together with the changing path length of the space-to-ground data link, complicates the array's operation. In particular, signals from the moving antenna must be temporally correlated with those from the ground telescopes before they can be combined to generate the interferometric data. However, such problems can be redressed by known techniques described in the literature. To facilitate description of the present invention, an array of fixed photon collectors is described--it being understood that techniques borrowed from this space radiotelescope prior art can be used to compensate for the dynamic effects introduced by placing one or more optical sensors into space.)
The interferometric principles employed in radio telescope arrays can likewise be extended to arrays of optical telescopes. Recent efforts in the optical interferometry domain include the Cambridge Optical Aperture Synthesis Telescope (COAST) and the Sydney University Stellar Interferometer (SUSI).
A further interferometric technique is amplitude interferometry as described, e.g., in Currie et al, "Four Stellar-Diameter Measurements by a New Technique: Amplitude Inteferometry," Astrophysical Journal, Vol. 187(1), Part 1, pp. 131,134 (Jan. 1, 1974). Amplitude interferometry is a variant of Michelson interferometry, designed to better measure stellar diameters in the presence of atmospheric turbulence.
All of the Michelson-based interferometry systems--whether radio or optical--rely on the wave conception of radiation, i.e. that light/radio waves exhibit localized maxima and minima which can be combined to constructively or destructively interfere and produce fringe patterns. This imposes on all such systems a high degree of physical precision (e.g. of sub-wavelength dimension) because received waves must be combined in known phase relationships in order for the resulting fringe patterns to have the desired meanings. At optical wavelengths, for examples, the lengths of connecting optical fibers must be physically or synthetically matched to within millionths of an inch. If such arrays are extended to space (c.f. NASA's New Millenium Interferometer: http://huey.jpl.nasa.gov/nmi/index.html), the locations of the space sensors must be ascertained to the nanometer--a specification which NASA acknowledges will require "a significant capability enhancement."
In contrast, the interferometry aspects of the present invention do not rely on these wave-based constructive/destructive interference forms of interferometry, with their attendant sub-wavelength tolerances. Instead, HBT interferometry is concerned with the correlation of stellar intensity signals over time. The accurate measurement of time is a far easier task than the attainment and maintenance of sub-wavelength physical tolerances. Moreover, the nature of correlation operations is forgiving of many temporal errors (i.e. sample records are time shifted as necessary to obtain optimum correlation, removing small errors in triggering times).
By eliminating the strict physical tolerances inherent in Michelson-based systems, costs are reduced and reliability is increased. Such detectors are thus well adapted for use in arrays whose baselines extend into space. Like Michelson systems, arbitrarily fine angular resolutions can be obtained by spacing the detectors accordingly.
Also disclosed in the following specification, and sharing the attribute of plural spaced detectors, is a more traditional optical imaging system (i.e. no interferometry). This system employs techniques described in the above-cited patents to characterize/model atmospheric turbulence through which a telescope is seeing.
In accordance with this further aspect of the present invention, these turbulence modeling techniques are used in an array of widely spaced small (e.g. three to twelve inch reflector) telescopes to characterize atmospheric turbulence above each telescope. (Each telescope is pointed at the same general region of the sky.) However, instead of using the resulting data to "unblur" each telescope's imagery, as in the patents' preferred embodiments, it is used to dynamically reposition--at essentially real time (e.g. 100 Hz repositioning)--the end of an optical fiber in each telescope's focal region. Such fibers are run from the plural telescopes to a central collection facility, where their opposite ends terminate in an image plane. This collected group of fiber terminations presents an image that can be viewed directly, or magnified/processed by traditional optics as desired. (Alternatively, the image can be sampled/recorded several times a second, and the resulting static images can be combined to synthesize a more accurate image.)
The foregoing arrangement yields extremely high angular resolution imagery (the exact resolution depends on the extent of the telescope array) while employing inexpensive optical components (e.g. the small component telescopes).
The foregoing and additional features and advantages will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying figures.